1. Field of the Invention
The present invention relates to the field of thermal processing, and has particular industrial utility in the field of laser thermal processing (LTP) of silicon substrates in the manufacturing of semiconductor devices.
2. Description of the Prior Art
LTP is a technique for manufacturing semiconductor devices such as integrated circuits or “ICs”. LTP involves irradiating a substrate, such as a doped semiconductor wafer, to rapidly bring the substrate surface from a relatively low temperature (e.g., 400° C.) to a relatively high temperature (e.g., 1,300° C.) so that the substrate bulk can pull the temperature down quickly. Such a rapid thermal cycle might be used, for example, to efficiently activate dopants in the substrate because only the material very close to the top surface of the substrate is heated to the relatively high temperature during irradiation.
As described in U.S. Pat. No. 6,747,245, one approach to LTP involves scanning a long, narrow laser beam back and forth across the substrate surface in a raster pattern. The amount of time the laser beam resides over a given point on the substrate surface is called the “dwell time”. Using this scanning approach, it is possible to achieve peak surface temperatures near 1350° C. with dwell times in the millisecond range. The result is a rapid thermal annealing of doped substrates that yields high activation levels with very little dopant diffusion. When used to fabricate transistor-based circuits, transistors with a sharply defined dopant profile and with a small sheet resistance are formed. LTP is capable of providing significantly lower sheet resistance values than possible using so-called Rapid Thermal Processing (RTP), which has dwell times equivalent to several seconds.
During LTP, it is usually necessary to pre-heat the substrate to 400° C. in order to ensure good coupling between the incident CO2 beam and the top of the substrate. Natural convection in the air space directly above the heated substrate surface leads to convective cells and time-varying air density changes that pass through the path of the annealing beam. These air density changes cause the path of the radiation beam to wander (dither). This, in turn, causes the amount of laser power delivered to the substrate surface to be distributed unevenly. Such variations can translate into unacceptable temperature non-uniformities at the substrate surface and lead to variations in IC performance.